Russia and Saudi Arabia haven't always been on the best of terms, with the Saudis long in the US sphere of influence. In what Russian Foreign Minister Sergey Lavrov called a "real turning point," however, Saudi King Salman has visited Russia, and come out with a $3 billion arms deal.
The $3 billion arms deal includes the purchase of S-400 Russian air defense systems, following Turkey's own recent purchase of the system. These acquisitions represent a growing interest among US allies in buying Russian military tech.
This probably isn't going to sit well with the Trump Administration, which has been trying to secure massive amounts of arms sales within the Middle East. Having to compete with Russia in future sales will likely be unwelcome news.